The much vilified Haynes Manual 63012(697) Mercedes-Benz Diesel 123 series, 1976 thru 1985 4&5 cyl has a section in chapter 7 showing some of the tear down of the inside of a manual transmission.
I bought mine used on Amazon.com (Ebay also has used ones) some times they are at the main library or even used book stores. You might be able to take a peek at one in the autoparts store and see if there is enough info for you buy one.
I have a 120 page book from ATSG (Automatic Transmission Service Group) that I bought on Ebay for my automatic Trans. the may also have books for manual transmissions.